DO NOT just "start it" to keep the battery charged. It will not really do that without taking the bike out for an hour or more ride. Starting the bike and running it for short periods CAN actually do more harm than good since the oil will not get up to high enough temperatures to drive off the condensing water inside the engine and oil tank. That water will combine with the combustion products in the oil and form acids. These are hard on the internal components of the engine and also hard on the oil.
Your best bet if you do not live in a year round riding area, is to winterize the bike by at least the following:
- Change the fluids (engine oil, chaincase lube, tranny lube)
- Fill the gas tank completely adding a fuel stabilizer
- Put a battery tender on the battery to maintain it
- Jack the bike up off the tires and put it on blocks or a lift
- Cover the bike with something like a sheet
You mentioned that you can't afford a Battery Tender. I see the Battery Tender Jr. on sale for nominally $20, so with S&H, it will be under $30. Think about getting one of these. It is well worth the investment since it will prolong your battery life.
TQ